Such a camera may use a mechanically scanned optical slit system. The particular application we concerned ourselves with is a camera to be used in a satellite orbiting a celestial body to be observed. The satellite camera has to provide as high resolution of image as feasible and has to have color-discriminating capability. Red (R), green (G), blue (B) and near infrared (NIR) responses to image are separated.
A CCD field imager, imaging an entire field at any time rather than just a slit or line in that field as a CCD line imager does, has to have an area array of photosensing sites. (In this document "photosensing site" refers to the location of a photosensing element, supposing photosensing not to be done within a CCD charge channel, or to the location at which charge carriers are collected in a CCD charge transfer channel, but supposing photosensing to be done in the semiconductive material in which the charge transfer channel is formed.) These sites are arranged in a field imager of field transfer type along each of parallelled charge transfer channels defining the columns of the array and are further arranged to be in rows. Each row is formed by the alignment of respective charge transfer stages in each charge transfer channel. So resolution in the direction transverse to line scan (and to rows of the array) is limited in a CCD field imager of field transfer type by the reciprocal of the minimum length of the charge transfer stage. A slight improvement is possible by staggering alternate charge transfer sites in the rows of the array. A problem is encountered in attempting to design a high-resolution field imager, as one seeks manyfold increase in the number of picture elements across the field in the direction transverse to line scan, particularly an increase beyond the number of minimum length charge transfer stages that will fit into each one of paralleled charge transfer channels extending across a semiconductor die of reasonable size. It appears to us to be difficult to extend column lengths in the array over adjoining semiconductor dies, while (as will be discussed later in this document) we were able to arrange for extending row lengths in a CCD imager by designing semiconductor dies that could be abutted with others of their like. Further, problems appear in the long-column arrays because of charge transport factor limitations. But, with transversal scanning being done by optical displacement of the image field relative to a line CCD imager, the minimum length of a charge transfer stage no longer imposes a limitation on resolution in the direction transverse to line scan. As we will reveal, this becomes important in arranging for color discrimination by the CCD imager.
Color discrimination properties have been provided many ways in the prior art to CCD field imagers, because of the interest in developing cameras for color television broadcasting or simpler color television cameras for home use. In broadcast cameras a color-discriminating optical beam splitter has been used to separate incoming radiation into spectral components for application to separate CCD field imagers. These imagers generate video signals responsive to the spectral components they respectively receive. This approach involves close optical registration of the photosensing sites on the various imagers to preserve image resolution, so it seemed best to us to avoid this approach in the high-resolution satellite camera.
CCD field imagers have also been used in conjunction with color-pattern optical filters, either of a contact type or of a type using relay optics. This practice of using color pattern filters with imaging devices arose during the development of single-tube color cameras, which used color-pattern optical filters to place stripes of differing colors on their targets in superposition with scan lines. The various color responses in electric signal from radiation received by the camera were subsequently sorted out by line selecting means following the camera tube. Color stripe filters with stripes parallel to line scan have not been used in CCD field imagers to our knowledge. In part this may be because the minimum length of charge transfer stages readily contructed on a semiconductor die of reasonable size, using current silicon-device design rules, just about fulfills the vertical resolution requirements of broadcast television standards, so the loss of vertical resolution incurred with a horizontal color stripe filter would complicate device design. A more important reason, perhaps, is to obtain superposed picture element samples for the various color responses one must resort to transversal low-pass spatial filtering involving the use of time delays of substantial length (i. e. of line scan time length). In any case, the color-pattern optical filters employed with CCD field imagers employ color patterns more complex than simple color stripes or employ color stripes in the direction transverse to line scan. The superposing of picture element samples in the various color responses by low-pass spatial filtering in the line scan direction, which is required with this latter type of color stripe filter, does not need long time delays.
If one attempts to perform patterned-color filtering with a line imager analogous to the filtering provided for one row of the CCD field imagers with patterned-color filters, the photosensing sites in the line imager receive radiation selectively spectrally filtered line segment by line segment. That is, the pattern of filtering corresponds to that provided by a color-stripe filter having stripes in the direction transverse to line scan. The color-pattern filtering, or color-stripe filtering transverse to direction of line scan, undesirably reduces the resolution of the imager in direction of line scan. Attempting to magnify the image optically before projecting it into a lengthened line imager, as a way of maintaining spatial resolution despite line image segmentation by the color filter, is unattractive to do. Even though buttable linear arrays on respective semiconductor dies can be used to accommodate lengthening of the line array, the increased capacitance associated with the gate electrodes of the line imager undesirably cuts down the speed with which charge samples can be moved in the imager and converted to output voltage samples.
We provide for color discrimination in our CCD line imager in a surprising way. We use an area array of photosensing sites--rather than a linear array--and we use a color-stripe filter with the color stripes running in the row direction of the CCD imager,--and in the direction of line scan, in the line imager output signal. In the direction transverse to line scan the line image passing through an optical slit filter is beam split or defocussed in the transverse direction, before passage through the color-stripe filter. That is, the same line image is replicated for application to each row of charge sensing sites in the area array, that application being made through respective stripes of the color-stripe filters. Each row of photosensing sites in the array receives a different spectral response stripe, but each different spectral response is to one of the replications of the same line image. An area array of rows of photosensing sites in which successive rows of the sites receive response to the same line image, rather than to successive line images in an area image, we term a "replicated line imager". Resolution in the direction of line scan is not compromised by such filtering.
The resolution in the transverse direction is determined by the optical slit filter, not by the imager, because we operate the area array as a replicated line imager. Scanning in the direction transverse to line scan still is accomplished by optical line selection. So we continue to avoid the limitation on resolution in this transverse direction imposed by the minimum length of a charge transfer stage in a field imager.
As noted above, the array of photosensing sites (i.e., the image, or A register) is arranged so the spectrally-filtered line images lie along the rows of the array, which are of extended length to provide high resolution in the direction of line scan. The columns of the array defined by the CCD charge transfer channels are short, facilitating the rapid transfer of line segment portions of the charge samples from the adjacent columns of the array into corresponding columns of a few-row field storage, or, B, register during relatively short transfer intervals between relatively long image integration times. After each transfer interval the charge samples can be advanced a row at a time in the field storage register with the row transferred out of the B register transferred in parallel into an output CCD line (or C) register for subsequent serial read-out to a charge sensing stage. This mode of operation is in many respects analogous to the field transfer method of charge sample extraction in field imagers first described by P. K. Weimer in U.S. Pat. No. 3,763,480 issued Oct. 2, 1973 and entitled "DIGITAL AND ANALOG DATA HANDLING DEVICES".
The very few rows and short columns in our replicated line imager area array make the charge sample extraction behave differently from a field imager with 1:1 or 3:4 height-to-width aspect ratio, where the Weimer charge sample extraction scheme is commonly employed. A single row advance transfers a number of charge samples in parallel that is a significantly larger fraction of the total number of charge samples transferred from the image register to the field storage register(s) in the field transfer interval. This permits a shorter field transfer interval relative to image integration interval, which reduces the need for interrupting the projection of image into the imager during field transfer in order to avoid transfer smear. Transfer smear is an unwanted source of crosstalk between the lines of charge samples that adversely affects the purity of the different color responses. Similarly, the time for transfer from the field storage register(s) to the output register(s) can be a substantially smaller fraction of the time between field transfer intervals. The proportion of time scan lines can be serially read out of the image output register(s) to the time they cannot is significantly higher than with a field imager, and this helps to keep the clocking rate of the output registers lower. Also the short columns of the image and field storage registers, despite the high resolution of the imager in the direction transverse to line scan, avoids the introduction of problems that would be associated with many signal charge transfers through those CCD registers.
Having solved the problem of providing high resolution in the direction transverse to line scan, the remaining problems with regard to a high-resolution CCD imager mostly concern obtaining the desired high resolution in the direction of line scan. The limitation on this resolution is imposed chiefly by the minimum length of the charge transfer stages in the output line (or C) register(s). Dividing this length (as determined by current silicon device design rules) into the distance between opposing parallel edges of a reasonable-size (by current standards) semiconductor die, we determined that the satellite camera would require a number of abutted such dies to obtain the number of picture elements per scan line required. The close abutting of photosensing elements on adjacent dies, required to maintain high resolution in the line scan direction, necessitate new layout techniques for situating the imager on the die.
Using abutted semiconductor dies with component line imagers, to provide a composite line image capable of sensing a longer line image at given resolution or of providing greater resolution for given line image length, means output registers on the dies need not operate at the high rate required of a single, long output register. That is, time-division-multiplexing of the output registers on the semiconductor dies can be eschewed in favor of spatial-multiplexing to cut the clocking rate required of the registers. In spatial multiplexing the imager output registers are read out in parallel at the same time.
Spatial-multiplexing can be carried further using a plurality of parallelly clocked output registers on the same semiconductor die for the component line imager. Then, too, the number of stages in each output (or C) register becomes of concern with regard to charge transfer efficiency being less than perfect and giving rise to transfer inefficiency noise. With the semiconductor die size we contemplated using, we decided that half the charge transfer channels used for transfer storage should be served by one output (or C) register; and the remaining half, by another output register.
Our initial concept of the layout of the replicated line imager on the semiconductor die, so abutting component line imagers to form a composite line imager would be facilitated was as follows. The rows of an image (or A) register would extend between the opposing edges of the die at least one of which would abut an adjoining die. The rows of a field storage (or B) register would also stretch between the edges, and its columns would continue from respective columns of the A register. The final charge transfer stages in the charge transfer channels making up the columns in a half of the B register closest to one edge of the die would connect by a fan-in structure to respective ones of the successive charge transfer stages in one of the two output (C) registers, and the other output register would have the final charge transfer stages in the other charge transfer channels of the B register connected to it analogously.
We conceived using a common electrometer, or charge sensing stage, centrally located between the opposing abuttable edges of the die, for both output registers, which output registers would be clocked for charge transfer in opposite directions towards the electrometer. The electrometer was to be a field effect transistor with gate connection to floating diffusions in each of the output registers. The central location of the electrometer would tend to keep elements away from the edges of the die that were adapted for abutting an adjoining die and reduce the amount of fan-in between the field storage register halves and their corresponsing output registers. Using a common electrometer for the two output registers, rather than separate ones, we believed would avoid congestion in the central portion of the semiconductor die.
While this concept of using plural output registers which charge transfer away from the opposing abuttable edges and towards charge sensing circuitry well within the confines of the semiconductor die top-surface boundary was retained in our subsequent camera development, we found a number of practical problems with our initial lay-out conception that tended to adversely affect the fabrication yield of the imager dies. Using a common electrometer stage for the two output registers on each die involved 90.degree. turns within 15 microns at the ends of those registers, which gave trouble. The busses carrying clocking signals to gate electrodes of the A and B registers had to cross the C registers, so avoiding clocking cross-talk was difficult. Bus routing problems were so severe that we could find no way to use anti-blooming drains, owing to the difficulty of making connections to the drains.
We found these problems could be avoided with a layout of the following type. The image (or A) register still extends from a first edge of the semiconductor die to a second opposing edge, but is divided into halves along a boundary midway between the first and second edges of the die. The A1 and A2 registers that are the respective halves of the A register are arranged for charge transfer in opposing directions to respective B1 and B2 field storage registers during charge transfer intervals. During subsequent image integration times when clocking is halted in the A1 and A2 registers, charge samples are clocked out of the B1 and B2 registers in the same directions as they were clocked in, being clocked in parallel into respective C1 and C2 output registers for conversion to serial format. Charge samples are serially clocked from the C1 and C2 registers towards the center of the die to be sensed in separate charge sensing circuits.